In some transmission channels, for example in the high- or medium-voltage systems with low system frequency (50 or 60 Hz), the significant disturbances consist of the various harmonics of the fundamental system frequency. Data transmission via such distribution systems must therefore take into consideration especially these harmonically related disturbances.
Several proposals have been made to suppress such harmonic disturbances during data transmission. Thus, a transmission method is known in which the data are transmitted by means of phase shift keying of a carrier frequency which is exactly in the center between two adjacent harmonics of the fundamental frequency, for example from U.S. Pat. No. 4,302,844 or from the article by J. T. Tengdin, "Field tests show AMRAC IV plc communication is promising", Modern Power Systems, June 1985, pages 57 et sequ.
The power spectrum of this phase shift keyed oscillation obeys a (sinx/x).sup.2 function symmetrically with respect to the carrier frequency. If then the symbol duration of the data symbols used for the phase shift keying is selected to be equal to twice the period duration of the fundamental disturbance frequency, the harmonics of this fundamental frequency are exactly located in the nulls of the signal power spectrum and can thus be suppressed on the receiver side by a suitable filter without impairment of the useful signal.
However, the known transmission method is restricted in its data rate to the comparatively low value of half the fundamental frequency because of the special choice of symbol duration with respect to the fundamental disturbance frequency which is not freely selectable. With a system frequency of 50 or 60 Hz, this corresponds to a data rate of 25 or 30 Baud, respectively.